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Developing a friction performance index for asphalt concrete mixtures


For the majority of asphalt mixtures produced in the U.S., their design is based solely on volumetric considerations. For a small but growing number of these mixtures, the design may also be based on structural performance, e.g., balanced mixture design (BMD). With the BMD concept and/or changes to the current design methodology, it is likely that mixtures will become finer, have an increased asphalt content, and potentially higher amounts of recycled materials. All of these changes will affect the functional characteristics; however, there does not currently exist a methodology to assess asphalt mixture designs to ensure that their functional performance characteristics. Worse, engineers lack a reliable, vetted procedure to consider functional performance at the mixture design stage. As such, state DOTs cannot actively engineer asphalt mixtures to exhibit sufficient functional and structural performance thresholds. Thus, they rely on ad hoc corrections when issues are identified.

This project will address this need by developing a procedure for state DOTs to evaluate their mixtures to either screen for those with poor functional performance or (ultimately) to design mixtures to avoid functional failures associated with material characteristics that do not balance functionality with structural durability. The experiences in developing balanced mixture design practices for rutting and fatigue demonstrate that failure to consider the consequences of potential test implementation (for example implementing a rutting test without including a cracking test) drives mixture behaviors in unexpected and unforeseen ways.

This research will address this problem by identifying the most effective, discriminating, and implementable test method or methods that state DOT’s can adopt and that other researchers can use to develop practical mixture design methodologies and also for quality assurance approaches. This project would be clearly aligned with the AASHTO Committee on Materials and Pavements, which is concerned with, among other things, asphalt materials testing, specification, and performance, as well as management. Given the connection between the surface condition of roadways and safety, this research also addresses the strategic goals for the Safety committee whose primary goal is to eliminate serious traffic crashes and save lives.


The objectives of this research are to develop a methodology, applicable to both the mixture design/selection procedures and quality assurance/quality control process, to assess the functional performance of asphalt concrete mixtures with respect to friction and macrotexture. A draft set of tasks that will likely need to be performed to achieve this goal are:

· Review existing methodologies for determination of asphalt mixture texture and friction and ongoing research in this domain. The evaluation should consider the role of friction and texture in safety (as defined by AASHTO and in NCHRP Report 37) and availability of datasets such as the KY continuous friction measurement data, asphalt mix design, and crash data.

· Select the most promising experimental method(s) to evaluate asphalt mixture performance with respect to friction and texture that can be applied during design and after construction.

· Develop the methodology for performance assessment. This methodology should be capable of evaluating asphalt mixtures macrotexture and microtexture but could consist of a combination of component material and composite tests and models.

· Establish initial performance thresholds for evaluation at the mixture design stage and for process control and quality assurance, and refinement in later research.

Develop a draft AASHTO specification(s) for review and submission as a provisional specification.


There is an urgent need for this work to address the unknown impacts on safety (i.e. friction qualities) of asphalt mixtures as the balanced mix design method is implemented by agencies. Developing a means of evaluating functional performance properties that can be applied both during design and post-production will allow functional performance and safety to be designed into asphalt mixtures, and will provide a means of ensuring that this performance is maintained through the QC/QA process


The results of this research will be used by state DOTs (specifically state DOT materials engineers), asphalt mixture designers, and asphalt paving contractors to ensure that asphalt mixtures are designed and produced with balanced structural and functional performance. Implementation activities may include:

· conference presentations (e.g. TRB Annual Meeting, RPUG, AASHTO), regional user-producer group presentations, BMD workshops, and webinars (e.g. TRB, AASHTO).

· supporting supplementary materials could include: a technical brief, short instructional videos, and whiteboard videos.

· a national workshop or webinar with invited participants for discussion on implementing the inclusion of functional assessment as part of mixture design and quality assurance/quality control processes.

· development of a follow-up effort to formulate guidelines to help state DOTs implement the functional assessment methodology into their mixture design and quality assurance processes.

· partnership with state DOTs currently participating in TPF-5(345/463) or similar studies.

Sponsoring Committee:AKP50, Pavement Surface Properties and Vehicle Interaction
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Shane Underwood and Boris Goenaga, North Carolina State University, 919-515-8632, shane.underwood@ncsu.edu
Date Posted:12/29/2021
Date Modified:03/15/2022
Index Terms:Friction factor, Index numbers, Asphalt concrete, Asphalt concrete pavements, Asphalt mixtures, Mix design,
Cosponsoring Committees: 
Safety and Human Factors

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